Exporting map models for games
Well, game environment is a very complex process. So you won't find the answer in a forum, is like asking "How to use blender?".

But I can give you a starting point.

Game environment could be divided in two main parts "Terrain" and "Assets". Terrain is the process of creating the soil, mountains, rivers, the topology (in landscape language not mesh language). Commonly it isn't created as a .obj, .fbx, etc... file. To do the landscaping usually you use heightmaps (Unreal Engine 4 https://www.youtube.com/watch?v=E5pHoWfpfYw , Unity https://www.youtube.com/watch?v=qpEMhWgJTZ4 ).

Assets use the .obj, .fbx, etc... file format. They are used to make buildings, add rocks, trees, improve the landscape (because heightmaps are only the foundation of the landscape). They are usually separated by object, a car is one file, a building is one file, a entire city isn't usually a single file because it cost a lot of computer resources, you usually create 5 buildings and repeat them.

Nevertherless there are modular games, but thats a whole new way of creating maps.

Also, usually the maps aren't builded straight away, first you draw like pics of the map (but this isn't necessary), a top view map of it with the main objects (buildings, mountains... Important trees (not all)). Then you design the heighmap and then put boxes and simple meshes to the map as a concept map and then start creating the assets.

In conclusion, maps are created in the game engine editor with the assets that you create in a modeling application (like blender).